Climate changes and recent glacier behaviour in the Chilean Lake District

Atmospheric temperatures measured at the Chilean Lake District (38 degrees-42 degrees S) showed contrasting trends during the second half of the 20th century The surface cooling detected at several meteorological stations ranged from -0.014 to -0.021 degrees C a(-1), whilst upper troposphere (850-300 gpm) records at radiosonde of Puerto Montt (41 degrees 26S/73 degrees 07'W) revealed warming between 0.019 and 0.031 degrees C a(-1). Regional rainfall data collected from 1961 to 2000 showed the overall decrease with a maximum rate of -15 mm a(-2) at Valdivia st. (39 degrees 38'S/73 degrees 05'W). These ongoing climatic changes, especially the precipitation reduction, seem to be related to El Nino-Southern Oscillation (ENSO) phenomena which has been more frequent after 1976. Glaciers within the Chilean Lake District have significantly retreated during recent decades, in an apparent out-of-phase response to the regional surface cooling. Moreover, very little is known about upper troposphere changes and how they can enhance the glacier responses. In order to analyse their behaviour in the context of the observed climate changes, Casa Pangue glacier (41 degrees 08'S/71 degrees 52'W) has been selected and studied by comparing Digital Elevation Models (DEMs) computed at three different dates throughout the last four decades. This approach allowed the determination of ice elevation changes between 1961 and 1998, yielding a mean thinning rate of -2.3 +/- 0.6 m a(-1). Strikingly, when ice thinning is computed for the period between 1981 and 1998, the resulting rate is 50% higher (-3.6 +/- 0.6 m a(-1)). This enhanced trend and the related area loss and frontal retreat suggests that Casa Pangue might currently be suffering negative mass balances in response to the upper troposphere warming and decreased precipitation of the last 25-30 yr, as well as debris cover would not prevent the glacier from a fast reaction to climate forcing. Most of recent glaciological studies regarding Andean glaciers have concentrated on low altitude changes, namely frontal variations, however, in order to better understand the regional glacier changes, new data are necessary, especially from the accumulation areas. (C) 2006 Elsevier B.V. All rights reserved.